Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier (CC) of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NDLRB*NRBsc subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as NDLsymb consecutive OFDM symbols in the time domain (e.g. 7 OFDM symbols) and NRBsc consecutive subcarriers in the frequency domain as exemplified in FIG. 4 (e.g. 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of NDLsymb*NRBsc resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)” (NPL 1), section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same NRBsc consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources. Similar assumptions for the component carrier structure apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (cells) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n*300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC). The characteristics of the downlink and uplink PCell are:                For each SCell the usage of uplink resources by the UE, in addition to the downlink ones is configurable; the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only.        The uplink PCell is used for transmission of Layer 1 uplink control information.        The downlink PCell cannot be de-activated, unlike SCells.        From UE perspective, each uplink resource only belongs to one serving cell.        The number of serving cells that can be configured depends on the aggregation capability of the UE.        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF.        The downlink PCell cell can change with handover (i.e. with security key change and RACH procedure).        Non-access stratum information is taken from the downlink PCell.        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure).        PCell is used for transmission of PUCCH.        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as “DL anchor carrier”. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data, thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
UL Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e. controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is for example the random access, i.e. when UE is performing initial access to a cell or for requesting uplink resources.
For the scheduled access Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel is called uplink grant channel in the following. A scheduling grant message contains at least information which part of the frequency band the UE is allowed to use, the validity period of the grant, and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e. there are no “per UE per RB” grants). Therefore the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA, there is no UE based transport format selection. The eNB decides the transport format based on some information, e.g. reported scheduling information and QoS info, and UE has to follow the selected transport format. In HSUPA the Node B assigns the maximum uplink resource, and UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided.        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme.        The UL reporting should allow fine granular buffer reports (e.g. per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users.        It should be possible to provide a minimum bit rate per radio bearer.        
As can be seen from above list one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
DRX (Discontinuous Reception)
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe.
DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 (NPL 2) Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e. the On-Duration periods at which the mobile node is active, and the periods where the mobile node is in a DRX mode.                On duration: duration in downlink sub-frames that the user equipment, after waking up from DRX, receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]        DRX inactivity timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]        DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]        DRX short cycle: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]        DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional. [1-16 subframes]        Long DRX Cycle Start offset: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0-subframe length of chosen cycle]]        
The total duration that the UE is awake is called “Active time”. The Active Time includes the on-duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HRQ RTT. Similarly, for the uplink the UE is awake at the subframes where uplink retransmission grants can be received, i.e. every 8 ms after initial uplink transmission until maximum number of retransmissions is reached. Based on the above, the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite).
The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e. is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time, during the DRX opportunity of time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e. only the long DRX cycle is used. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next on-duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Precoding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active time, type-O-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to On Duration.
Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be as long as the paging DRX used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval, i.e. long DRX intervals may have more relaxed requirements (for more details see further below). When DRX is configured, periodic CQI reports can only be sent by the UE during “active-time”. RRC can further restrict periodic CQI reports so that they are only sent during the on-duration.
FIG. 7 discloses an example of DRX. The UE checks for scheduling messages (indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “on duration” period of either the long DRX cycle or the short DRX cycle depending on the currently active cycle. When a scheduling message is received during an “on duration”, the UE starts an “inactivity timer” and monitors the PDCCH in every subframe while the Inactivity Timer is running During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a “short DRX cycle timer”. The short DRX cycle may also be initiated by means of a MAC Control Element. When the short DRX cycle timer expires, the UE moves into a long DRX cycle.
In addition to this DRX behaviour, a “HARQ Round Trip Time (RTT) timer” is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least “HARQ RTT” subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
Uplink Power Control
Uplink transmission power control in a mobile communication system serves an important purpose: it balances the need for sufficient transmitted energy per bit to achieve the required Quality of Service (QoS) against the need to minimize interference to other users of the system and to maximize the battery life of the mobile terminal. In achieving this purpose, the role of the Power Control (PC) becomes decisive to provide the required SINR while controlling at the same time the interference caused to neighbouring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, the 3GPP has adopted for LTE the use of Fractional Power Control (FPC). This new functionality makes users with a higher path-loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
Detailed power control formulae are specified in LTE for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRSs) (for further details on the power control formulae, see for example 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, version 8.8.0 (NPL 3) or 9.1.0, section 5.1, available at http://www.3gpp.org and incorporated herein by reference). The formula for each of these uplink signals follows the same basic principles; in all cases they can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signaled by the eNodeB, and a dynamic offset updated from subframe to subframe.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all user equipments in the cell (measured in dBm) and a UE-specific offset, and an open-loop path-loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the MCS and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications asΔTF 
where TF stands for “Transport Format”) allows the transmitted power per resource block to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes: accumulative TPC commands (available for PUSCH, PUCCH and SRS) and absolute TPC commands (available for PUSCH only). For the PUSCH, the switch between these two modes is configured semi-statically for each UE by RRC signaling, i.e. the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level.
Power Headroom Reporting
In order to assist the eNodeB to schedule the uplink transmission resources to different user equipments in an appropriate way, it is important that the user equipment can report its available power headroom to eNodeB.
The eNodeB can use the power headroom reports to determine how much more uplink bandwidth per sub-frame a user equipment is capable of using. This helps to avoid allocating uplink transmission resources to user equipments which are unable to use them in order to avoid a waste of resources.
The range of the power headroom report is from +40 to −23 dB (see 3GPP TS 36.133, “Requirements for support of radio resource management”, version 8.7.0 (NPL 4), section 9.1.8.4, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The negative part of the range enables the user equipment to signal to the eNodeB the extent to which it has received an UL grant which would require more transmission power than the UE has available. This would enable the eNodeB to reduce the size of a subsequent grant, thus freeing up transmission resources to allocate to other UEs.
A power headroom report can only be sent in sub-frames in which a UE has an UL transmission grant. The report relates to the sub-frame in which it is sent. The headroom report is therefore a prediction rather than a direct measurement; the UE cannot directly measure its actual transmission power headroom for the sub-frame in which the report is to be transmitted. It therefore relies on reasonably accurate calibration of the UE's power amplifier output.
A number of criteria are defined to trigger a power headroom report. These include:                A significant change in estimated path loss since the last power headroom report        More than a configured time has elapsed since the previous power headroom report        More than a configured number of closed-loop TPC commands have been implemented by the UE        
The eNodeB can configure parameters to control each of these triggers depending on the system loading and the requirements of its scheduling algorithm. To be more specific, RRC controls power headroom reporting by configuring the two timers “periodicPHR-Timer” and “prohibitPHR-Timer”, and by signalling “dl-PathlossChange” which sets the change in measured downlink pathloss to trigger a power headroom report.
The power headroom report is send as a MAC Control Element. It consists of a single octet where the two highest bits are reserved and the six lowest bits represent the 64 dB values mentioned above in 1 dB steps. The structure of the MAC Control Element for the Rel-8 power headroom report is shown in FIG. 8.
The UE power headroom PH [dB] valid for sub-frame i is defined by the following equation (see section 5.1.1.2 of 3GPP TS 36.213):PH(i)=PCMAX−{10·log10(MPUSCH(i))+P0_PUSCH(j)+a(j)·PL+ΔTF(i)+f(i)}   (Equation 1)
The power headroom is rounded to the closest value in the range [40; −23] dB with steps of 1 dB. PCMAX is the total maximum UE transmit power (or total maximum transmit power of the user equipment) and is a value chosen by the user equipment in the given range of PCMAX_L and PCMAX_H based on the following constraints:PCMAX_L≤PCMAX≤PCMAX_H PCMAX_L=min(PEMAX−ΔTc,PPowerClass−MPR−AMPR−ΔTc)PCMAX_H=min(PEMAX,PPowerClass)PEMAX is the value signaled by the network, and MPR, AMPR (also denoted as A-MPR) andΔTc are specified in 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception”, version 8.7.0 (NPL 5), section 6.2 available at http//www.3gpp.org and incorporated herein by reference.
MPR is a power reduction value, the so-called Maximum Power Reduction, used to control the Adjacent Channel Leakage Power Ratio (ACLR) associated with the various modulation schemes and the transmission bandwidth.
A-MPR is the additional maximum power reduction. It is band specific and it is applied when configured by the network. Therefore, PCMAX is UE implementation specific and hence not known by eNB.
Uplink Power Control for Carrier Aggregation
One main point of UL Power control for LTE-Advance is that a component carrier specific UL power control is supported, i.e. there will be one independent power control loop for each UL component carrier configured for the UE. Furthermore power headroom is reported per component carrier.
In Rel-10 within the scope of carrier aggregation there are two maximum power limits, a maximum total UE transmit power and a CC-specific maximum transmit power. RAN1 agreed at the RAN1#60bis meeting that a power headroom report, which is reported per CC, accounts for the maximum power reduction (MPR). In other words, the power reduction applied by the UE is taken into account in the CC-specific maximum transmission power PCMAX,c (c denotes the component carrier). As already mentioned before, the purpose of MPR/A-MPR is to allow the mobile device to lower its maximum transmission power in order to be able to meet the requirements on signal quality, spectrum emission mask and spurious emissions.
As already mentioned before the purpose of values MPR/A-MPR is to allow the mobile device to lower its maximum transmission power in order to be able to meet the requirements on signal quality, spectrum emission mask and spurious emissions.
In addition to MPR and A-MPR in Release 10 the so called power management MPR, also referred to as P-MPR, was introduced in order to account for multi-RAT terminals which may have to limit their LTE total output power, particularly when simultaneous transmission on another RAT is taking place. Such power restrictions may arise, for example from regulations on Specific Absorption Rate (SAR) of radio energy into a user's body or from out-of-band emission requirements that may be affected by the inter-modulation products of the simultaneous radio transmissions. The P-MPR is not aggregated with MPR/A-MPR, since any reduction in a UE's maximum output power for the latter factor helps to satisfy the requirements that would have necessitated P-MPR.
Considering now the additional power management MPR (P-MPR), the UE configures its nominal maximum transmission power PCMAX, i.e. the maximum transmission power available for the UE, according to the following equations:PCMAX_L≤PCMAX≤PCMAX_H PCMAX_L=MIN{PEMAX−ΔTc,PPowerClass−max(MPR+A-MPR,P-MPR)−ΔTc}PCMAX_H=MIN{PEMAX,PPowerClass}
For the case of carrier aggregation, the PCMAX becomes PCMAX,c the component-carrier specific maximum transmission power. Essentially the configured maximum output power on serving cell c shall be set within the following bounds:PCMAX_L,c≤PCMAX,c≤PCMAX_H,c 
Two different deployments are to be considered, one where aggregated carriers are within the same frequency band, and also the case where carriers of different frequency bands are aggregated.
For intra-band contiguous carrier aggregation:PCMAX_L,c=MIN{PEMAX,c−ΔTC,c,PPowerClass−MAX(MPRc+A-MPRc+ΔTIB,c,P-MPRc)−ΔTC,c}
For inter-band carrier aggregation:PCMAX_L,c=MIN{PEMAX,c−ΔTC,c,PPowerClass−MAX(MPRc+A-MPRc+ΔTIB,c,P-MPRc)−ΔTC,c}PCMAX_H,c=MIN{PEMAX,c,PPowerClass}
PEMAX,c is the value given by IE P-Max for serving cell c in TS36.331
For inter-band carrier aggregation, MPRc and A-MPRc apply per serving cell c, i.e. there is a separate MPR and A-MPR per serving cell. For intra-band contiguous carrier aggregation, MPRc=MPR, and A-MPRc=A-MPR. P-MPRc accounts for power management for serving cell c. For intra-band contiguous carrier aggregation, there is one power management term for the UE, P-MPR, and P-MPRc=P-MPR.
For carrier aggregation with two UL serving cells, the total configured maximum output power PCMAX shall be set within the following bounds:PCMAX_L_CA≤PCMAX≤PCMAX_H_CA For intra-band contiguous carrier aggregation,PCMAX_L_CA=MIN{10·log10ΣpEMAX,c−ΔTC,PPowerClass−MAX(MPR+A-MPR+ΔTIB,c,P-MPR)−ΔTC}PCMAX_H_CA=MIN{10·log10ΣpEMAX,c,PPowerClass}where pEMAX,c is the linear value of PEMAX,c which is given by RRC signalling (for details see TS 36.331 incorporated herein by reference).
For inter-band carrier aggregation with up to one serving cell c per operating band:PCMAX_L_CA=MIN{10·log10ΣMIN[pEMAX,c/(ΔtC,c),pPowerClass/(mprc-a-mprc-ΔtC,c-ΔtIB,c),pPowerClass/(p-mprc·ΔtC,c)],PPowerClass}PCMAX_H_CA=MIN{10·log10ΣpEMAX,c,PPowerClass}where pEMAX,c is the linear value of PEMAX,c which is given by TS 36.331. MPRc and A-MPRc apply per serving cell c and are specified in subclause 6.2.3 and subclause 6.2.4 of TS36.101, respectively, also incorporated herein by reference. mprc is the linear value of MPRc. a-mprc is the linear value of A-MPRc. P-MPRc accounts for power management for serving cell c. p-mprc is the linear value of P-MPRc.
Further information about the definition of CC-specific maximum transmission power respectively the UE total maximum transmission power can be found in TS36.101, incorporated herein by reference.
Different to Rel-8/9 for LTE-A the UE has also to cope with simultaneous PUSCH-PUCCH transmission, multi-cluster scheduling, and simultaneous transmission on multiple CCs, which requires larger MPR values and also causes a larger variation of the applied MPR values compared to Rel-8/9.
It should be noted that the eNB does not have knowledge of the power reduction applied by the UE on each CC, since the actual power reduction depends on the type of allocation, the standardized MPR value and also on the UE implementation. Therefore, the eNB does not know the CC-specific maximum transmission power relative to which the UE calculates the PHR. In Rel-8/9 for example UE's maximum transmit power PCMAX can be within some certain range as described above.PCMAX_L≤PCMAX≤PCMAX_H 
Due to the fact that the power reduction applied by the UE to the maximum transmit power of a CC is not known by eNB it was agreed to introduce in Rel-10 a new power headroom MAC control element, which is also referred to as extended power headroom MAC control element. The main difference to the Rel-8/9 PHR MAC CE format, is that it includes a Rel-8/9 power headroom value for each activated UL CC and is hence of variable size. Furthermore it not only reports the power headroom value for a CC but also the corresponding PCMAX,c (maximum transmit power of CC with the index c) value. In order to account for simultaneous PUSCH-PUCCH transmissions, UE reports for PCell the Rel-8/9 power headroom value which is related to PUSCH only transmissions (referred to type 1 power headroom) and if the UE is configured for simultaneous PUSCH-PUCCH transmission, a further Power headroom value, which considers PUCCH and PUSCH transmissions, also referred to as type 2 power headroom.
In order to be able to distinguish at the eNB side whether the maximum transmission power was reduced due to MPR/A-MPR power reduction or caused by applying a P-MPR, a one bit indicator, also referred to as P-bit, was introduced in the extended power headroom MAC CE. More in particular the UE sets P=1 if the corresponding maximum transmission power (PCMAX,c) would have had a different value if no power backoff due to power management (P-MPR) had been applied. Essentially this P bit is used by the eNB to remove the PHR reports, which are affected by P-MPR, from the MPR-learning algorithm in the eNB, i.e. eNB stores in an internal table which MPR value the UE uses for certain resource allocations.
For further details on the extended power headroom MAC Control element illustrated in FIG. 9, see for example 3GPP TS 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 10)”, version 10.0.0 (NPL 2), section 6.1.3.6a, available at http://www.3gpp.org and incorporated herein by reference.
Type-1 power headroom can also be reported for subframes where there is no actual PUSCH transmission. This special PHR is also referred to as virtual PHR. In such cases, 10 log10(MPUSCH(i)) andΔTF,c(i)
in the expression of the power headroom report shown above are set to zero. Values for the pathloss (PL), received TPC commands f(i) and other CC specific constants (P0_PUSCH(j), a) are available for the UL CC, even without UL data transmission:PHvirtual,c(i)=PCMAX,H,c−{P0_PUSCH(j)+a(j)+PLc+f(i)}
This can be seen as the power headroom assuming a default transmission configuration corresponding to the minimum possible resource assignment (M=1) and the modulation-and-coding scheme associated withΔTF,c(i)=0 dB.
The carrier-specific maximum transmission power{tilde over (P)}CMAX,c(i)
is computed assuming
MPR=0 dB
A-MPR=0 dB
P-MPR=0 dB
ΔTc=0 dB.
Essentially,{tilde over (P)}CMAX,c(i)
is equal to PCMAX_H,c=MIN{PEMAX,c, PPowerClass}.
Similar to Type-1 power headroom reporting, the Type-2 power headroom can also be reported for subframes in which no PUSCH and/or PUCCH is transmitted. In that case a virtual PUSCH and/or PUCCH transmit power is calculated, assuming the smallest possible resource assignment (M=1) andΔMCS=0 dBfor PUSCH andh(nCQI,nHARQ,nSR),ΔF_PUCCH(F),ΔTxD(F′)set to 0 for PUCCH. Further details about the computation of power headroom can be found in TS36.213, incorporated herein by reference.
Small Cells
Explosive demands for mobile data are driving changes in how mobile operators will need to respond to the challenging requirements of higher capacity and improved Quality of user Experience (QoE). Currently, fourth generation wireless access systems using Long Term Evolution (LTE) are being deployed by many operators worldwide in order to offer faster access with lower latency and more efficiency than 3G/3.5G system. Nevertheless, the anticipated future traffic growth is so tremendous that there is a vastly increased need for further network densification to handle the capacity requirements, particularly in high traffic areas (hot spot areas) that generate the highest volume of traffic. Network densification—increasing the number of network nodes, thereby bringing them physically closer to the user terminals—is a key to improving traffic capacity and extending the achievable user-data rates of a wireless communication system.
In addition to straightforward densification of a macro deployment, network densification can be achieved by the deployment of complementary low-power nodes respectively small cells under the coverage of an existing macro-node layer. In such a heterogeneous deployment, the low-power nodes provide very high traffic capacity and very high user throughput locally, for example in indoor and outdoor hotspot positions. Meanwhile, the macro layer ensures service availability and QoE over the entire coverage area. In other words, the layer containing the low-power nodes can also be referred to as providing local-area access, in contrast to the wide-area-covering macro layer.
The installation of low-power nodes respectively small cells as well as heterogeneous deployments has been possible since the first release of LTE. In this regard, a number of solutions have been specified in recent releases of LTE (i.e., Release-10/11). More specifically, these releases introduced additional tools to handle inter-layer interference in heterogeneous deployments. In order to further optimize performance and provide cost/energy-efficient operation, small cells require further enhancements and in many cases need to interact with or complement existing macro cells. Such solutions will be investigated during the further evolution of LTE-Release 12 and beyond. In particular further enhancements related to low-power nodes and heterogeneous deployments will be considered under the umbrella of the new Rel-12 study item (SI) “Study on Small Cell Enhancements for E-UTRA and E-UTRAN”. Some of these activities will focus on achieving an even higher degree of interworking between the macro and low-power layers, including different forms of macro assistance to the low-power layer and dual-layer connectivity. Dual connectivity implies that the device has simultaneous connections to both macro and low-power layers.
Some deployment scenarios assumed in this study item on small cell enhancements will be discussed below. In the following scenarios, the backhaul technologies categorized as non-ideal backhaul in TR 36.932 are assumed.
Both ideal backhaul (i.e., very high throughput and very low latency backhaul such as dedicated point-to-point connection using optical fiber) and non-ideal backhaul (i.e., typical backhaul widely used in the market such as xDSL, microwave, and other backhauls like relaying) should be studied. The performance-cost trade-off should be taken into account.
A categorization of non-ideal backhaul based on operator inputs is listed in the table below:
TABLE 1Priority Backhaul Latency (1 is the Technology(One way)Throughputhighest)Fiber Access 110-30 ms10 M-10 Gbps1Fiber Access 2 5-10 ms100-1000 Mbps2Fiber Access 3 2-5 ms50 M-10 Gbps1DSL Access15-60 ms 10-100 Mbps1Cable25-35 ms 10-100 Mbps2Wireless  5-35 ms10 Mbps-100 Mbps typical,1Backhaulmaybe up to Gbps range
Fiber access which can be used to deploy Remote Radio Heads (RRHs) is not assumed in this study. HeNBs are not precluded, but not distinguished from Pico eNBs in terms of deployment scenarios and challenges even though the transmission power of HeNBs is lower than that of Pico eNBs. The following 3 scenarios are considered.
Scenario #1 is illustrated in FIG. 10 and is the deployment scenario where macro and small cells on the same carrier frequency (intra-frequency) are connected via a non-ideal backhaul. User are distributed both for outdoor and indoor.
Scenario #2 is illustrated in FIGS. 11 and 12 and refers to a deployment scenario where macro and small cells on different carrier frequencies (inter-frequency) are connected via a non-ideal backhaul. User are distributed both for outdoor and indoor. There are essentially two different scenarios #2, referred herein as 2a and 2b, the difference being that in scenario 2b an indoor small cell deployment is considered.
Scenario #3 is illustrated in FIG. 13 and refers to a deployment scenario where only small cells on one or more carrier frequencies are connected via a non-ideal backhaul link.
Depending on the deployment scenario, different challenges/problems exist which need to be further investigated. During the study item phase such challenges have been identified for the corresponding deployment scenarios and captured in TS 36.842; more details on those challenges/problems can be found there.
In order to resolve the identified challenges which are described in section 5 of TS36.842, the following design goals are taken into account for this study in addition to the requirements specified in TR 36.932.
In terms of mobility robustness:                For UEs in RRC_CONNECTED, Mobility performance achieved by small cell deployments should be comparable with that of a macro-only network.        
In terms of increased signaling load due to frequent handover:                Any new solutions should not result in excessive increase of signaling load towards the Core Network. However, additional signaling and user plane traffic load caused by small cell enhancements should also be taken into account.        
In terms of improving per-user throughput and system capacity:                Utilising radio resources across macro and small cells in order to achieve per-user throughput and system capacity similar to ideal backhaul deployments while taking into account QoS requirements should be targeted.        
Dual Connectivity
One promising solution to the problems which are currently under discussion in 3GPP RAN working groups is the so-called “dual connectivity” concept. The term “dual connectivity” is used to refer to an operation where a given UE consumes radio resources provided by at least two different network nodes connected via a non-ideal backhaul. Essentially, the UE is connected with both a macro cell (macro eNB) and small cell (secondary or small eNB). Furthermore, each eNB involved in dual connectivity for a UE may assume different roles. Those roles do not necessarily depend on the eNB's power class and can vary among UEs.
Since the study item is currently at a very early stage, details on dual connectivity are not decided yet. For example the architecture has not been agreed on yet. Therefore, many issues/details, e.g. protocol enhancements, are still open currently. FIG. 14 shows an exemplary architecture for dual connectivity. It should be only understood as one potential option; the present disclosure is not limited to this specific network/protocol architecture but can be applied generally. The following assumptions on the architecture are made here:                Per bearer level decision where to serve each packet, C/U plane split. As an example UE RRC signalling and high QoS data such as VoLTE can be served by the Macro cell, while best effort data is offloaded to the small cell.        No coupling between bearers, so no common PDCP or RLC required between the Macro cell and small cell.        Looser coordination between RAN nodes.        SeNB has no connection to S-GW, i.e. packets are forwarded by MeNB.        Small Cell is transparent to CN.        
Regarding the last two bullet points, it should be noted that it's also possible that SeNB is connected directly with the S-GW, i.e. S1-U is between S-GW and SeNB. Essentially, there are three different options w.r.t. the bearer mapping/splitting:                Option 1: S1-U also terminates in SeNB; depicted in FIG. 15a         Option 2: S1-U terminates in MeNB, no bearer split in RAN; depicted in FIG. 15b         Option 3: S1-U terminates in MeNB, bearer split in RAN; depicted in FIG. 15c         
FIGS. 15a-c depict those three options taking the downlink direction for the U-Plane data as an example. For explanation purposes, option 2 is mainly assumed for this application, and is the basis for FIG. 14 too.
In addition to the discussion on the splitting of the U-plane data as depicted in FIGS. 15a-c, different alternatives have been discussed for the user plane architecture too.
A common understanding is that, when the S1-U interface terminates at the MeNB (FIG. 15b,c), the protocol stack in the SeNB must at least support RLC (re-)segmentation. This is due to the fact that RLC (re-)segmentation is an operation that is tightly coupled to the physical interface (e.g. MAC layer indicating size of the RLC PDU, see above), and when a non-ideal backhaul is used, RLC (re-)segmentation must take place in the same node as the one transmitting the RLC PDUs.
Shortcomings of Prior Art Power Control
As explained in the previous sections, small cells and dual connectivity are a recent development and still pose several problems that need to be addressed in order to allow for an efficient system.
In the dual connectivity scenarios as explained above simultaneous uplink transmissions by the UE to both the MeNB and SeNB (also referred to as dual Tx) are supported for Release 12. There are two independent schedulers, one residing in the MeNB and the other one residing in the SeNB, which each schedule uplink transmission for the UE independently from one another. More in particular, uplink resource allocations scheduled in one cell are not known in the other cell. In other words, the MeNB scheduler is not aware of uplink scheduling decisions made by the SeNB, and vice versa.
For said reason, there is an increased probability that the UE will be power limited, i.e. that the UE total maximum transmission power is exceeded when two uplink transmission are scheduled with too much power.
This is illustrated in FIG. 16, which shows a power-limited situation where two uplink transmissions are scheduled for the UE, one to the MeNB and one to the SeNB. As apparent therefrom, the simultaneous uplink transmissions exceed to the total maximum UE transmit power, for which reasons power scaling is performed by the UE for the uplink transmissions so as to keep the total power used for the two transmissions below the total maximum UE transmit power. Power scaling in turn reduces the scheduling efficiency and performance.